Coupler assemblies for stations supplying a gas for patient use are well known in the art. It is also known to provide means for securing the parts together to avoid an inadvertent separation and cutting off of the gas supply. Such prior art structures are found in G. L. Rogers U.S. Pat. No. 2,908,511 and B. C. Cranage U.S. Pat. Nos. 3,441,046 and 3,477,105 which are incorporated herein by reference. Each of these patents discloses a latching mechanism for preventing inadvertent separation of the coupler assembly with all of the latching parts being inside of a base member in the latching position. Each of these patents discloses a spring actuated latch holding mechanism located inside the base member and engaging a slot in a latch pin which is moved axially into the interior of the base. In each case the operation of the latch cannot be observed to insure satisfactory operation. Further, any malfunction of the latch is difficult to repair requiring disassembly of the base member. The latching mechanism of Rogers U.S. Pat. No. 2,908,511 is complex since it employs a plunger and other mechanism within the base to activate and inactivate the latch responsive to the position of a member inserted into the base. In both the Cranage patents cited above the actuation of a member protruding from the base is required in order to release the latch. By way of marked contrast the coupler assembly of the invention is all exterior of the base with the means for releasing the latch incorporated with the hose terminal fittings for ease of operation with one hand. While the latch of the invention is highly reliable, in the case of any malfunction it can be quickly replaced without disturbing the base. The coupler assembly of the invention is also advantageous in that it insures that the desired two gases will be supplied simultaneously since it secures together the hose terminal fittings. Further, it insures that each hose is connected to the desired gas by using different diameter plugs and sockets. It is simpler in construction and less expensive than the prior art structures.